The enclosure you set up for a beetle depends almost entirely on whether you're housing larvae or adults. They have different needs, and trying to do both in one box usually means doing both badly. So here's how to get it right from the start.
Larvae: it's mostly about the substrate
Beetle larvae (grubs) spend their entire lives buried in substrate. They eat it, burrow through it, and eventually pupate in it. The enclosure is secondary to what goes inside it. A plain plastic tub with a few small ventilation holes works perfectly. It doesn't need to be pretty.
What matters is depth. For something like Pachnoda marginata (sun beetle) larvae, you want at least 12-15 cm of substrate. Larger species, such as rhinoceros beetles or Mecynorrhina, need 20 cm or more. The larvae need enough room to move through the substrate without constantly hitting the bottom of the tub.
The substrate itself has to be fermented hardwood flake soil. Regular compost, potting soil, or garden earth won't work. Beetle larvae feed on the white-rot fungi that break down hardwood during fermentation. Without it, they slowly starve even though they appear to be eating. It's one of the most common ways people lose larvae, and it's completely avoidable.
You can buy ready-made flake soil from invert suppliers, or make your own by fermenting oak or beech woodchips with water over several months. Buying it is easier and more consistent, especially when you're starting out.
Larval enclosure sizing
Individual L3 (final instar) larvae of larger species like Dynastes hercules or Trypoxylus dichotomus can each need a container of roughly 5-8 litres. Smaller flower beetle larvae can be kept communally in a larger tub, as long as the substrate volume is generous enough that they're not competing. As a rough guide, allow about 2 litres of substrate per Pachnoda larva.
The tub needs a lid with small ventilation holes. Not wide-open mesh, because you'll lose all your humidity. Just enough airflow to prevent things going anaerobic. Punch a few holes in the lid with a heated needle or small drill bit.
Adult beetle enclosures
Once your beetles have eclosed (emerged from their pupal cells as adults), the setup changes completely. Adults are active. They climb, fly, mate, and eat fruit. They need space and structure.
A glass or acrylic terrarium works well. Something around 30x30x30 cm is fine for a group of sun beetles. Bigger species need bigger enclosures, obviously, but beetles don't need the floor space of, say, a reptile. Height matters too, because many flower beetles are decent flyers and will use the vertical space.
Put 5-8 cm of substrate on the bottom. For adults, this doesn't need to be high-quality flake soil. A mix of coconut fibre and organic topsoil is fine. If you've got breeding females in there, add a deeper section of flake soil (10-15 cm) in one corner or a separate laying container, because they need fermented substrate to lay eggs into.
Furnishing and feeding
Add some branches and cork bark for climbing. Flower beetles are active during the day and will use anything you put in there. A piece of fruit (banana, apple, or mango) on a small dish gives them something to eat. Remove it before it goes mouldy, which usually means swapping it every couple of days. Beetle jelly pots are a cleaner alternative and last longer.
Don't use a water dish. Beetles can drown in even shallow water. They get all their moisture from food and from the ambient humidity in the enclosure. Speaking of which, mist lightly every day or two. You want the substrate slightly damp but not sodden.
Temperature
Most commonly kept beetle species are tropical or subtropical. Aim for 22-26C for the majority. Some species, like Dynastes hercules, do better at the warmer end (24-27C) during the larval stage but benefit from slightly cooler conditions as adults.
A heat mat on one side of the enclosure, controlled by a thermostat, is the standard approach if your room doesn't stay warm enough. Don't put the mat underneath a larval tub, because the larvae will burrow down into the warmest substrate and can overheat. Side-mounted is safer.
Ventilation
Too much ventilation dries the enclosure out. Too little and you get mould and stagnant air. For adult enclosures, a mesh strip or a few rows of holes near the top of the enclosure usually does the job. For larval tubs, minimal ventilation is fine because the substrate holds moisture and the grubs aren't fussy about airflow.
If you're seeing a lot of mould on the substrate surface of an adult enclosure, it's usually an airflow problem rather than a humidity problem. Increase ventilation slightly before you reduce misting.
One thing people forget
Label your tubs. If you're keeping multiple species or multiple batches of larvae, they all look the same after a few weeks. Date of birth, species, and substrate change dates on a piece of tape will save you from confusion later. It's boring admin but it stops you losing track of what's what, especially once you're maintaining a dozen containers.